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44227-G9
Competition between Chaotic Mixing and Self-Organizing of Colloids and Nanoparticles
The goal is to understand how flows of various topologies generate chaotic advection that interplays with shear-induced migration of suspensions. Shear-induced migration of particles results from multibody hydrodynamic interactions of particles that generate a negative normal stress in the fluid and may play a significant role in processing petroleum suspensions where solids may have practically the same physical and chemical properties as the fluids. In previous studies in simple 1D geometries (tubes, Couette cells, and parallel plates), particles were observed to migrate toward regions of low shear where these hydrodynamic interactions are minimized. In pressure-driven flows, particles migrate toward center of the channel and the local increased viscosity causes the velocity profile to deviate from parabolic to a flatter profile in the center with higher shear near the walls. Inducing 3D flows breaks symmetries of these simple flows, and has been shown to greatly enhance dispersion of Newtonian liquids through generation of chaotic advection.
Our primary hypothesis driving this research is channel geometry producing 3D flows (including chaotic flows) can be used to direct self-organization within suspension flows. Thus far, we have been able to show significant deviations in concentration based on the channel geometry producing flows that essentially can be mapped as 1D, 2D and 3D topologies. An abbreviated summary of results to date on three geometries, straight (1D), symmetric herringbone (2D), and staggered herringbone (3D), are described here. Details of these results vary for various concentrations and flow rates, examined from concentrations of 0.1 to 0.4 and flow rates from 2 uL/hr to 50 uL/hr ranging in Reynolds numbers from 10-7 < Re < 10-5 and Péclet numbers from 5000 < Pe < 125,000, however each result is in general similar to that described here.
Investigations in 1D, 2D, and 3D flows show greatly varying concentration profiles. A stable concentration profiles result in each channel geometry, however instead of a single concentrated band forming in simple channels, 2D and 3D flows produce multiple bands. In the 2D flow, the location of these bands corresponds with the formation of two recirculating regions. In the 3D flow, three bands are formed, however the location of these bands is unclear. Both 2D and 3D flows result in enhanced transport in the transverse direction and closer proximity of particles to the sidewalls of the channels.
Many questions remain regarding these segregation profiles. The mechanism presented here is competition between shear-induced migration and advection in 2D and 3D flows, however other possibilities exist. Particle-wall interactions may also play a significant roll in the observed pattern formation. Near wall hydrodynamic interactions can drive particles from the sidewalls toward the center of the channel. Although these interactions only drive migration within a few particle diameters of the wall, the resulting particle-free fluid is injected into the bulk of the fluid. This could explain particle-depleted region in the center of the 2D flow show above. Likewise, in a chaotic flow, the near wall interactions are spread throughout the channel, and could result in the observed segregation profile without shear-induced migration. Experiments at significantly lower concentrations need to be performed to determine whether the mechanism is particle-particle or particle-boundary interactions. Continued investigations of these phenomena and those of binary suspensions to investigate the result of nanoparticle-microsphere interactions are underway.
